The overall goal of this project is to develop and commercialize a simple cell counter for routine enumeration of CD4 cells for management of HIV-infected individuals. HIV-1 mediated CD4 cell destruction is the central immunologic feature of HIV-1 infection. Thus, the CD4 count is a critical measurement in initial disease staging, in monitoring antiretroviral therapy and in managing primary and secondary prophylaxis for opportunistic infections. Flow cytometry is the current standard-of-care for CD4 cell counting. Unfortunately, flow is a central lab-based technique, and sample cold chain and transport, equipment, and operational costs render the technique cost-prohibitive in limited resource settings where HIV prevalence is highest. A simple, point-of-care (POC) CD4 counting tool will fundamentally change HIV-1 management. The first phases of this project have yielded a low-cost prototype instrument that delivers fast, accurate, and reliable CD4 counts for whole blood specimens from both HIV-infected individuals and healthy donors. Building on this success, we propose to complete the technical development by minimizing required user interactions with the device, driving toward an FDA CLIA-waived system. The specific aims are: 1. Simplify the sample processing protocol and design and build a cartridge/instrument prototype for which the only user interaction is whole blood addition. We will investigate and develop assay reagent combinations (e.g., stain, lysis buffer, etc.), reagent packaging, and modified disposable cartridge and instrument designs to reduce the number of liquid transfer steps in the assay. The development goal is a CLIA-waived (or equivalent) system. 2. Incorporate in-cartridge quality controls that confirm correct function and user operation. We will develop a combination of calibration kit components and on-cartridge quality control features, including count calibrators, fluorescent stain indicators and image quality software flags. 3. Demonstrate the accuracy and usability of the CD4 counting system in clinical settings. Field evaluations of the prototype system will be performed at HIV-specific clinics affiliated with the University of California, San Diego (UCSD) Medical Center and in Maputo, Mozambique. Supporting this effort we have established strong clinical collaborations with global health thought leaders. Co-investigators Dr. Robert Schooley and Dr. Constance Benson at UCSD provide critical input on device requirements, and facilitate laboratory evaluations. At the conclusion of this project, the utility, value, and unique capabilities of the MBio point of care C4 cell counter will be strongly established, and the final steps in commercialization, i.e., productin instruments, clinical trials, regulatory submission, and marketing will be accelerated.